The present invention relates to an overcurrent protective device for switching elements, and, more particularly, to an overcurrent protective device for switching elements suitable for use in an inverter device that uses a battery of a vehicle as a power supply.
An inverter device drives household electric appliances using a battery installed in a vehicle as a power supply. Since some of such appliances may cause excessively high load acting on the inverter device, an overcurrent protective device for switching elements is provided in the inverter device. An example of the overcurrent protective device is illustrated in FIG. 6. As shown in the drawing, the overcurrent protective device includes a current detecting circuit 45 that detects a current flowing in each of switching elements 41, 42, 43, 44, which form an inverter. If an overcurrent is detected, all of the switching elements 41 to 44 are turned off, including the switching element(s) that should be maintained in an ON state if the overcurrent were not, detected.
After the switching elements 41 to 44 are turned off due to the overcurrent that has been detected by the current detecting circuit 45, the current flowing in each switching element 41 to 44 decreases. This reduces the voltage detected by the current detecting circuit 45, and the overcurrent is canceled. The switching elements that should be maintained in an ON state if the overcurrent is not detected are thus turned on. Therefore, if the excessively high load is continuously applied to the inverter, the switching elements 41 to 44 are repeatedly turned on and off in a relatively short period.
This causes the switching elements 41 to 44 to switch at high frequencies, increasing switching loss. To solve this problem, an overcurrent protective device that prevents a switching frequency from increasing when an overcurrent protective circuit is in operation has been proposed (see, for example, Japanese Laid-Open Patent Publication No. 2002-354659).
Also, an output current limiting system for a motor driving inverter has been proposed (see, for example, Japanese Laid-Open Patent Publication No. 5-137392). The system suppresses oscillation of a motor current in an overcurrent protective device that turns off all the switching elements of an inverter when an overcurrent is detected. The system also prevents occurrence of surge voltage. Specifically, as illustrated in FIG. 7, the system turns off only one of a switch Sa1 and a switch Sb2 if an overcurrent is caused when the switches Sa1, Sb2 are both held in an ON state. When the switch Sa1 is turned off, a reactive current is caused in a closed circuit that is defined by a point P, a motor resistor R, an inductance L, the switch Sb2, and a diode Da2. The reactive current is thus consumed by the motor resistor R. When the switch Sb2 is turned off, a reactive current is caused in a closed circuit that is defined by the point P, the motor resistor R, the inductance L, a diode Db1, and the switch Sa1. This causes the reactive current to be consumed by the motor resistor R.
Further, there are cases in which switching of an inverter in overcurrent protective operation is performed at a frequency higher than the upper limit of audio frequency, which is 20 kHz. For example, if the switching frequency is set to 40 kHz in the overcurrent protective device of Japanese Laid-Open Patent Publication No. 2002-354659, which prevents the switching frequency from increasing when the overcurrent detecting circuit is in operation, the inverter may cause disturbing noise when all of the switching elements of the inverter are turned off. More specifically, since the inverter carries out regenerative operation while switching of the switching elements is continuously performed at the frequency of 40 kHz, the current flows toward the plus terminal of an electrolytic capacitor C and that of a power supply (not shown), as indicated by the arrow of FIG. 6. This causes a rapid fall in the current waveform, as illustrated in FIG. 5. By the time the overcurrent is canceled, the voltage has decreased in correspondence with the time for which the switching elements have been maintained in off states for overcurrent protection. Therefore, controlling of the switching elements is resumed in accordance with a maximally increased duty. The fall of the output current thus becomes more rapid than the rise, greatly changing the duty. This apparently causes a ripple waveform current of approximately 13 kHz, one third of 40 kHz, thus generating whine.
In the system of Japanese Laid-Open Patent Publication No. 5-137392, when an overcurrent is detected, energy of an output coil is consumed by a load. That is, instead of the regenerative operation, a flywheel operation is carried out as an overcurrent protective operation. In this case, the fall of the current when the switching elements are maintained in an OFF state becomes moderate, suppressing a great change of the duty. Therefore, the output current varies at 40 kHz, thus preventing generation of whine. However, if it is impossible to consume the energy of the output coil due to, for example, a short circuit caused in the output, the peak of the current may become excessively great, thus damaging the switching elements.